(1) FIELD OF THE INVENTION
This invention relates to methods and apparatus for conducting electrical measurements of epithelial tissue. More particularly, the present invention relates to methods and apparatus for making electrical measurements upon membrane samples.
(2) PRIOR ART
The technology associated with making electrical measurements upon epithelial tissue has conventionally involved employing a test chamber. Pieces of epithelia are positioned between two sections of a cylindrical chamber. Aerated fluid is circulated through the test chamber to maintain the tissue in a living state. Each chamber section conventionally has at least four ports. Two ports connect with conduits for circulating aerating fluids through the chamber. In addition, there are two ports which provide electrical communication with exterior metal electrodes. Gel-electrolyte "salt bridges", which are tubes filled with agar gel saturated with a salt solution, connect with metal electrodes. The metal electrodes are disposed in beakers which contain a salt solution. In the conventional electrical measurement technology to which the invention relates, each of the chamber sections is connected to a air of electrodes in the circulating fluid system. Because of the potentially toxic effect of metal and metallic compounds upon the test subject, metallic electrodes are usually not directly introduced into the chambers. The gel-filled salt bridge ordinarily retards the transport of metal or metal products to the tissue to be tested. Four electrodes are normally employed in the conventional testing apparatus. One electrode in each of the chamber sections measures the voltage between the chamber sections, and a second pair of electrodes is employed to establish current flow through the experimental tissue disposed between the chamber sections.
Recent advances in the technology of epithelial culture and growth have involved growing the epithelia in plastic cell culture cups having a diameter which may range from approximately 5 to 30 millimeters. The culture cups include paper or porous bottom structures upon which the epithelia cells are induced to grow in a skin-like layer. The technology of electrical testing of the epithelia has accordingly adapted to the widespread usage of culture cups by the use of apparatus and methods which involve inserting the tissue culture cup with the tissue to be tested inside the conventional test chamber. However, the insertion of a tissue culture cup into a conventional testing chamber presents a number of here-to-for unsolved problems. First, the apparatus is relatively expensive and difficult to fabricate. Secondly, the apparatus conventionally is only adapted to accommodate a single sample at a time thereby resulting in serial sample analyses and subsequent serialization which is extremely time consuming. In addition, the relatively long gel-filled salt bridge tubes which lead to the electrodes are unwieldy, time consuming to prepare and become easily dislodged from the testing apparatus. Also, because the long salt bridge tubes introduce a relatively high electrical resistance to the current pathway, a relatively high voltage is required in order to force the current through the membrane sample. Use of a high voltage is undesirable because it is inconvenient electronically, most modern test equipment requiring low voltage at the input terminals, and because it presents a shock hazard.